Case Reports
Keeping an eye on the ring: COMS plaque loading optimization for improved dose conformity and homogeneity

Purpose: To improve tumor dose conformity and homogeneity for COMS plaque brachytherapy by investigating the dosimetric effects of varying component source ring radionuclides and source strengths.

Material and methods: The MCNP5 Monte Carlo (MC) radiation transport code was used to simulate plaque heterogeneity-corrected dose distributions for individually-activated source rings of 14, 16 and 18 mm diameter COMS plaques, populated with ¹⁰³Pd, ¹²⁵I and ¹³¹Cs sources. Ellipsoidal tumors were contoured for each plaque size and MATLAB programming was developed to generate tumor dose distributions for all possible ring weighting and radionuclide permutations for a given plaque size and source strength resolution, assuming a 75 Gy apical prescription dose. These dose distributions were analyzed for conformity and homogeneity and compared to reference dose distributions from uniformly-loaded ¹²⁵I plaques. The most conformal and homogeneous dose distributions were reproduced within a reference eye environment to assess organ-at-risk (OAR) doses in the Pinnacle3 treatment planning system (TPS). The gamma-index analysis method was used to quantitatively compare MC and TPS-generated dose distributions.

Results: Concentrating > 97% of the total source strength in a single or pair of central 103Pd seeds produced the most conformal dose distributions, with tumor basal doses a factor of 2-3 higher and OAR doses a factor of 2-3 lower than those of corresponding uniformly-loaded ¹²⁵I plaques. Concentrating 82-86% of the total source strength in peripherally-loaded ¹³¹Cs seeds produced the most homogeneous dose distributions, with tumor basal doses 17-25% lower and OAR doses typically 20% higher than those of corresponding uniformly-loaded ¹²⁵I plaques. Gamma-index analysis found > 99% agreement between MC and TPS dose distributions.

Conclusions: A method was developed to select intra-plaque ring radionuclide compositions and source strengths to deliver more conformal and homogeneous tumor dose distributions than uniformly-loaded ¹²⁵I plaques. This method may support coordinated investigations of an appropriate clinical target for eye plaque brachytherapy.